of coherent state and how its can interfere, such explanation looks likely. It is assumed that collisions must destroy coherent states. And really the experi-.
arXiv:physics/0401051v1 [physics.optics] 12 Jan 2004
Coherency is the ether of XXI century. V.A.Kuz’menko Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow region, 142190, Russian Federation. Abstract The concept of coherent states in explanation of a nature of nonlinear phenomena in optics will be inevitably replaced by the concept of inequality of forward and reversed transitions. PACS number: 42.50Gy, 42.50.Hz
The concept of ether exists for many centuries. But in the twentieth century its necessity practically comes to the end and for most physicists this idea remains only as a historical curious thing. In substantial degree similar future, obviously, wait for the concept of coherency, which now widely used for explanation of nonlinear phenomena in optics. Formerly such effects usually are explained as a nonlinear response of substance on high intensity laser radiation [1]. In fact this is not an explanation, but only a certification of result: if the substance has high nonlinear optical susceptibility coefficient of second, third or higher order, then the high intensity laser radiation will give corresponding nonlinear effect. The Maxwell-Bloch equations or the rotating wave approximation is usually used for mathematical description of nonlinear phenomena. This mathematical model in many cases gives good description of nonlinear effects, but it does not have clear physical sense and the physical origin of nonlinear phenomena should be explained independently. The concept of coherency now plays a main role in explanation of origin of laser-driven nonlinear optical effects. It is supposed, that laser radiation creates a so-called coherent states. Those states have specific properties and can interfere. As a result the nonlinear phenomena appear, like as coherent population trapping [2], electromagnetically induced transparency [3], amplification without inversion [4,5], etc. Although it is not sufficiently clear what is the nature of coherent state and how its can interfere, such explanation looks likely. It is assumed that collisions must destroy coherent states. And really the experimentalists observe, that collisions destroy the discussed nonlinear effects. As a result, the concept of coherency become all pervasive in the field of laser and quantum optics [6]. However, a skilled theoretical analysis had discovered the inward defect in the coherency concept [7,8]. It has been shown that the inability to measure the absolute phase of an electromagnetic field prohibits the existence of coherent states. In fact it means that the using of concept of coherent states for explanation of nonlinear phenomena does not have physical sense. This result 1
is a shock for experimentalists. It robs and nothing gives in exchange for their favorite toy. The experimentalists can not agree with such situation. They exert strong pressure on theorists. As a result, some attempts to revise this theoretical conclusion appear. The work [6] contains excellent description of the situation as a whole and makes attempt to save the concept of coherent states by searching the weak point in arguments of works [7,8]. The goal of this note is not to discuss the arguments. The goal is to point out that we do not need to save the concept of coherent state. There exists alternative concept, which better suit for explanation of nature of nonlinear effects. We keep in mind the concept of inequality of forward and reversed optical transitions, which corresponds to concept of time invariance violation in electromagnetic interactions [9]. The orthodox point of view claims, that the time invariance violation is absent in electromagnetic interactions. This point of view does not have experimental proofs, but it has a long historical tradition [10]. In contrast, the experimental proofs for the opposite point of view appear recently. First of all this is an experimental study of forward and reversed transitions in optics. Seemingly, it is very simple to test the invariance of a photon absorption process: we should measure the parameters of photon absorption process (spectral width and cross-section) and compare its with the measured parameters of the reversed stimulated emission process. But really the situation is not so simple. Both processes take place simultaneously. Furthermore, the spectral width of laser radiation or optical transition are usually connected accordingly with the pulse length and life time of excited states toward a spontaneous emission. All this reasons make so entangled situation, that reliable measurement and comparison of the necessary parameters of forward and reversed processes become perfectly problematic. However, in one unique for present day case this problem is easily overcame. This is the case of the so-called line wings in the absorption spectrum of polyatomic molecules [9]. This physical object has unusual properties: long lifetime of excited states toward a spontaneous emission is combined with extremely large homogeneous spectral width of optical transition. In this case simple pump-probe experiments in a molecular beam conditions clearly show, that the spectral width for the reversed transition is in several order of magnitude smaller, than those for the forward transition, accordingly, the cross-section of the reversed transition is in several order of magnitude grater, than the crosssection of the forward transition [11]. For present day there is the first and the most clear and reliable experimental proof of time invariance violation in optics. Other experimental proof was obtained recently in the study of interaction between light and metallic planar chiral nanostructures [12,13]. Besides that, the quite formal fresh sight analysis of high order nonlinear phenomena in optics shows theirs violation of time reversal symmetry [14]. Time invariance violation is very good foundation for explanation of physical origin of nonlinear effects. Inequality of forward and reversed transitions endows atoms and molecules the properties of memory about the initial state 2
and aspiration to return in this state. This properties look like very similar to those properties of coherent states: the memory also must be destroyed by collisions. This properties allow to give simple and clear explanation of physical origin of most nonlinear phenomena [9,15]. So, we believe, that the concept of inequality of forward and reversed transitions will inevitably replace in nonlinear optics the concept of coherent states. Because of: 1) it much better explain the nature of most nonlinear phenomena, 2) in contrast to the concept of coherent states, it has a real physical base [16,17], 3) in contrast to the concept of coherent states, it has direct experimental proofs [9,12,13]. There is possible other way also. Because of the myths (like as the ether or the coherent states) have not usually a strict definition, we can preserve the brand of the concept, but substitute for it contents. Coherent states can be defined as a specific states of atoms and molecules after forward optical transitions. However, such variant may be inconvenient because of it makes some mishmash for readers (such situation exists now in rather similar case with the concept of the ”quasicontinuum” of vibration states of polyatomic molecules [18]). In conclusion, the orthodox point of view is very strong now. The experimentalists are afraid to work in the new direction. They need to be aimed by the theorists. But the theorists use their efforts now for saving the hopeless idea. So, dear colleagues, look around and let us begin to work in the right direction at last.
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[11] C.Liedenbaum, S.Stolte, and J.Reuss, Chem.Phys. 122, 443 (1988). [12] A.Papakostas, A.Potts, D.M.Bagnall, S.L.Prosvirnin, H.J.Coles, and N.I.Zheludev, Phys.Rev.Lett. 90, 107404 (2003). [13] A.S.Schwanecke, A.Krasavin, D.M.Bagnall, A.Potts, A.Zayats, and N.I.Zheludev, E-print, cond-mat/0307056. [14] M.Xiaochun, E-print, physics/0308037. [15] V.A.Kuzmenko, E-print, phesics/0310090. [16] A.T.Holster, New J.Phys. 5, 130 (2003). [17] J.S.M.Ginges and V.V.Flambaum, E-print, physics/0309054. [18] V.A.Kuz’menko, E-print, physics/0204003.
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